Bark Keeps Surface Cool Under the Sun

Biological Strategy

Plants

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Protect From Light

Access to sunlight is crucial to living systems because it’s the primary energy source for life. However, too much sunlight in the form of ultraviolet radiation (UV) can cause damage to living tissues. Therefore, living systems have strategies to filter out some or all UV radiation. For example, some plants that live in areas exposed to long periods of direct sunlight have reflective surfaces (such as white hairs or powder) that reflect UV light.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Bark of trees keeps surface cool by minimizing absorption of solar light and maximizing thermal emission

Image: DMF Prazeres /

Eucaliptus bark

Image: Derek Ramsey, Chanticleer Garden / Wikimedia commons /

A picture of the bark on the trunk of the Paperbark Mapleen (Acer griseum en ) tree. Photo taken at the Chanticleer Garden where it was identified.

Image: Derek Ramsey, Chanticleer Garden / Wikimedia commons /

Photograph of the detail in the bark of the Flowering Dogwooden (Cornus florida en First_Lady). Photo taken at the Tyler Arboretum where it was identified.

Image: DMF Prazeres /

Pine bark

“It was found that tree barks have optimized their reflection of incoming sunlight between 0.7 and 2 [microns]. This is approximately the optical window in which solar light is transmitted and reflected by green vegetation. Simultaneously, the tree bark is highly absorbing and thus radiation emitting between 6 and 10 [microns]. These two properties, mainly provided by tannins, create optimal conditions for radiative temperature control. In addition, tannins seem to have adopted a function as mediators for excitation energy towards photo-antioxidative activity for control of radiation damage.” (Henrion and Tribusch 2009:98)

“Optimal reflection of incoming radiation is not the only condition for keeping surfaces cool. The second condition, optimal emission at a temperature that the object reaches (approximately 40 ºC) has also been demonstrated here. Barks, through tannin and , efficiently absorb radiation between 6 and 10 [microns] (Figs. 10, top and 12), which means that they emit radiation equally well in this wavelength region. It allows efficient thermal emission into the cool space via an optical atmospheric window between 7.5 and 13 [microns] (Fig. 6). Since leaves also have to cool down efficiently and therefore also have to emit radiation in this infrared optical window, it can be speculated that cellulose and tannins have been evolved, adopted, and applied because of their radiation properties. This could have been at least one of several evolutionary factors. As already mentioned the optical properties of tree barks are of course not the only factor that has been considered by evolution with respect to the heat balance of the tree bark. Many tree barks show a paper-like structure with sheets peeling off, generating highly heat-insulating, trapped air spaces between them (e.g. birch, paper-bark tree). Other tree barks have developed a very rough surface that produces a lot of shadowed areas amongst the illuminated ones [11]. Such a morphology is known to stimulate convection of air, which then transports away heat from the bark surface. An additional fact or that has been considered by evolution was, as already mentioned, the structure of tree stems and branches. They typically have a round profile, minimizing the surface with respect to the volume. For that reason, comparatively little solar heat can come in from the outside.” (Henrion and Tribusch 2009: 104-5)

Last Updated October 14, 2016

References

“Optimal reflection of incoming radiation is not the only condition for keeping surfaces cool. The second condition, optimal emission at a temperature that the object reaches (approximately 40 ºC) has also been demonstrated here. Barks, through tannin and cellulose, efficiently absorb radiation between 6 and 10 [microns] (Figs. 10, top and 12), which means that they emit radiation equally well in this wavelength region. It allows efficient thermal emission into the cool space via an optical atmospheric window between 7.5 and 13 [microns] (Fig. 6). Since leaves also have to cool down efficiently and therefore also have to emit radiation in this infrared optical window, it can be speculated that cellulose and tannins have been evolved, adopted, and applied because of their radiation properties. This could have been at least one of several evolutionary factors. As already mentioned the optical properties of tree barks are of course not the only factor that has been considered by evolution with respect to the heat balance of the tree bark. Many tree barks show a paper-like structure with sheets peeling off, generating highly heat-insulating, trapped air spaces between them (e.g. birch, paper-bark tree). Other tree barks have developed a very rough surface that produces a lot of shadowed areas amongst the illuminated ones [11]. Such a morphology is known to stimulate convection of air, which then transports away heat from the bark surface. An additional fact or that has been considered by evolution was, as already mentioned, the structure of tree stems and branches. They typically have a round profile, minimizing the surface with respect to the volume. For that reason, comparatively little solar heat can come in from the outside.” (Henrion and Tribusch 2009: 104-5)

Journal article

Optical solar energy adaptations and radiative temperature control of green leaves and tree barks

Solar Energy Materials and Solar Cells | 11/11/2008 | Wolfgang Henrion, Helmut Tributsch

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